Owners with AGM battery -- what is the typical max voltage (after resting)?

[sajohnson]

After a full charge cycle with a Battery Tender (BT) or similar?

After a long drive?

100% SOC is supposed to be 13.0V, but I've only seen that once after using a BT.

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[incavulator]

I did a long drive (~200 miles) on Friday. Then another ~40 miles on Saturday. That's pretty rare, as the BS is not my primary vehicle. On all occasions, when I return home, it goes back in the garage and gets put on the Battery Tender because it often sits for a couple weeks at a time.

I just measured voltage after removing the Battery Tender. It read 13.18V.

EDIT: One more thing I should mention, with these IBS (intelligent battery sensors), I don't think SOC is calculated purely from static voltage. I believe they are using other inputs/calculations as well. A battery can have a surface charge which will make it look good until you apply a load, and then it drops like a rock. I don't know the intricate details, but I'm pretty sure it's more complicated than just voltage at key on or at the end of a drive cycle.

 

[sajohnson]

I did a long drive (~200 miles) on Friday. Then another ~40 miles on Saturday. That's pretty rare, as the BS is not my primary vehicle. On all occasions, when I return home, it goes back in the garage and gets put on the Battery Tender because it often sits for a couple weeks at a time.

I just measured voltage after removing the Battery Tender. It read 13.18V.

EDIT: One more thing I should mention, with these IBS (intelligent battery sensors), I don't think SOC is calculated purely from static voltage. I believe they are using other inputs/calculations as well. A battery can have a surface charge which will make it look good until you apply a load, and then it drops like a rock. I don't know the intricate details, but I'm pretty sure it's more complicated than just voltage at key on or at the end of a drive cycle.

13.18V is about what I see with the BT still connected, or maybe immediately after disconnecting it.

What is the resting voltage, after the AGM sits for a couple hours with no charge and no load?

The reason I ask is that most AGM SOC tables look like this:

I've only seen 13V once. It's typically 12.65 > 12.7V after resting.

 

[sajohnson]

For example, my wife drove 180 miles r/t 2 days ago, almost all interstate.

The Badlands has be sitting since then. I just checked the battery and it's at 12.67V. Perfect for a FLA battery, but low for an AGM -- about 87% SOC.

 

[incavulator]

I'll pull the BT off now and will check voltage in a couple hours and report back.

 

[sajohnson]

I just disconnected the BT.

* Before removing it the voltage was 13.05V.
* A few minutes after disconnecting it, the voltage had dropped to 12.91V.

I'll check it again after a couple hours also.

 

[incavulator]

12.93V after just over 2 hours off the BT.

 

[sajohnson]

12.78V after 2:45.

12.93 to 12.78V is a significant difference.

Out of curiosity I just checked the Deltran BT site to see if their newer maintainers and chargers have a dedicated AGM mode. They still combine FLA and AGM (lithium is separate):
https://www.batterytender.com/products/battery-tender®-12v-5-amp-power-tender®-battery-charger

 

[alank2]

Wasn't there a forscan parameter that determines what SOC maximum that the BMS will charge to?

 

[sajohnson]

Wasn't there a forscan parameter that determines what SOC maximum that the BMS will charge to?

Good question. I wouldn't be surprised if there is. To be honest, I haven't looked into Forscan.

Here's some info from our friends at Google:

"Yes, charging a starting battery to an 80% state of charge (SOC) is standard and safe, but doing it exclusively will cause gradual degradation. Lead-acid batteries require periodic "absorption" charging (the final 20%) to prevent permanent sulfation and to maintain their maximum capacity. [1, 2, 3, 4, 5]

The Mechanics of AGM Charging

• • Bulk Phase: A smart charger will rapidly push current to bring an AGM battery up to about 80% SOC. This stage is safe and efficient.
  • Absorption Phase: Getting from 80% to 100% requires the charger to maintain a constant, elevated voltage (typically 14.4V – 14.8V for 12V AGMs) while naturally tapering off the current. [1, 2]

The Damage Factor

• • Why the last 20% matters: If you routinely stop charging at 80%, the active lead material will begin to harden (known as hard sulfation). This causes permanent capacity loss. [1, 2]
  • Alternator limitations: In a standard vehicle, your alternator will primarily focus on replacing the surface charge (getting you back to that 80% mark after an engine start). It rarely achieves a full 100% on short trips. [1, 2]
  • Smart Charger Requirement: To avoid damage, you should connect the battery to a microprocessor-controlled smart charger on a monthly basis to complete the absorption phase and equalize the cells."
  Here's some info on the BT charging profile:
• 
  "Deltran Battery Tenders utilize a 4-step automatic charging profile specifically tailored for 12-volt AGM starting batteries. This process adjusts voltage and amperage to safely charge and maintain the battery without overcharging: [1, 2, 3, 4, 5]
  1. Initialization: The charger checks the battery to ensure it can safely accept a charge (typically requiring at least 3 volts). [1]
  2. • Bulk Charge: The charger delivers a constant maximum current until the battery's voltage rises to about 14.4 to 14.6 volts. [1, 2]
     • Absorption Mode: The charger maintains a constant voltage of 14.4 to 14.6 volts. As the battery absorbs the charge, the current gradually decreases, ensuring the battery reaches 100% capacity. [1]
     • Float Maintenance Mode: The charger switches to a steady, low voltage (usually around 13.2 to 13.4 volts). This safely maintains the battery at a full charge for long-term storage. [1, 2, 3, 4]
  For more specifics, you can review the Battery Tender FAQ or Battery Tender Blogs."
  
  
• We don't drive the BL much, so I typically have the BT connected. If the above is accurate, we should be in good shape.
  
  

 

[sajohnson]

A couple hours ago it was 12.71V

We ran it just long enough to get it into the garage. I'll check it again in a little while.

 

[Stircrazy]

13.18V is about what I see with the BT still connected, or maybe immediately after disconnecting it.

What is the resting voltage, after the AGM sits for a couple hours with no charge and no load?

The reason I ask is that most AGM SOC tables look like this:

I've only seen 13V once. It's typically 12.65 > 12.7V after resting.

here is a open circuit SOC chart

 

[sajohnson]

here is a open circuit SOC chart

That's interesting. I've never seen OCV specified. That should always be the case with SOC charts -- it's understood that the voltage is measured at rest with no load.

The lower voltage makes me feel better about our AGM battery, but almost all other SOC charts I've seen look like the one in post #3 above.

 

[sajohnson]

A couple hours ago it was 12.71V

We ran it just long enough to get it into the garage. I'll check it again in a little while.

A bit more than 'a little while'...

I just checked and the battery measured 12.63V.

I went ahead and connected a different BT -- same exact model, just a different unit to see if that makes any difference in the final charged voltage.

 

[Stircrazy]

That's interesting. I've never seen OCV specified. That should always be the case with SOC charts -- it's understood that the voltage is measured at rest with no load.

The lower voltage makes me feel better about our AGM battery, but almost all other SOC charts I've seen look like the one in post #3 above.

the one above is basicly if you shut the car off and check the voltage, before any rest period. with all the stuff going on in todays vehicles thats usaly what your going to get unless you take the time to disconect the battery and let it settle for a couple hours then measure it

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